Consequences of Conventional versus Organic farming on Soil Carbon: Results from a 27‐Year Field Experiment

Abstract

Organic farming practices are regarded as being beneficial for the environment by promoting soil quality and sequestering soil organic carbon (SOC). We studied SOC dynamics in the long‐term field experiment DOK in Switzerland. The experiment compares three organically fertilized treatments under conventional (CONFYM), bioorganic (BIOORG), and biodynamic (BIODYN) management, and two systems with (CONMIN) or without (NOFERT) mineral fertilizer. We analyzed measured SOC time series from 1977 to 2004 and applied soil fractionation, radiocarbon dating, and modeling with the carbon model RothC. The SOC declined significantly in most parcels, but was not systematically different between systems. Initial SOC contents correlated with soil texture and were identified as being important with respect to the change rate. The SOC loss was at the expense of mineral‐associated carbon whereas the more labile fractions increased. The overall decline was explained by reduced carbon inputs since commencement of the experiment and was most pronounced in NOFERT and CONMIN. The model satisfactorily simulated the dynamics of most of the treatments for both initialization with equilibrium runs or measured SOC fractions. Carbon loss in CONFYM was not fully captured by the model. Composition of organic fertilizers depended on the particular management, and a model adjustment of their relative stability improved the match between model and measurements. Model runs without management effects indicated that the observed increase in temperatures at the experimental site does not induce a change in SOC. Overall, the study does not support a benefit of organic farming on SOC contents compared with conventional systems with manure.

Organic Farming and Soil Carbon Sequestration: What Do We Really Know About the Benefits?

“…Composted manure is typical for DYN, and, given that composting stabilizes organic matter such as raw manure, nominal equal C inputs may have differential effects on SOC. For the DOK experiment a loss of 21% C for 'conventional' manure versus 39% for composted manure prior to field application has been estimated (Leifeld et al 2009). Thus, the actual field balance without considering CO 2 produced off-site becomes biased.…”

“…In a paired t test, the difference in SOC content between DYN and ORG was significant. Only one experiment in a Californian vineyard showed no difference in SOC between ORG and DYN (Reeve et al 2005),whereas systematic differences were found in two controlled, long-term experiments at temperate sites in Germany (Raupp 2001) and Switzerland (Fließbach et al 2007, for lower fertilization level, and Leifeld et al 2009, for higher fertilization level). In the study of Raupp (2001), loss of SOC relative to CON was smaller in DYN than in ORG for low-and medium-level manure application and DYN was the only system that gained SOC under high organic fertilization.…”

“…In contrast to Raupp (2001), the pretreatment of manure differed between ORG (matured manure) and DYN (composted manure). In a recent publication on that experiment, Leifeld et al (2009) argued that differences between the different organic systems were mostly due to differences in initial SOC and not due to the practice itself. Together, at current there is no unequivocal evidence for a positive effect of DYN on SOC stocks relative to ORG, and thus more research with properly designed and documented experiments is needed.…”

“…Three experiments comprising six datasets compared ORG and DYN plots (Raupp 2001;Reeve et al 2005;Fließbach et al 2007;Leifeld et al 2009). Among these, ORG lost -0.74% (±0.10) whereas biodynamic systems lost only 0.40% (±0.13).…”

“…This may be due to factors other than effects of current management, for example legacy of previous land-use where underlying long-term trends in SOC may modify or even superimpose management-induced trends (Leifeld et al 2009;Smith et al 2007). Also, a given management practice or type of management may include various individual activities which jointly affect SOC levels.…”